1. Field of the Invention
The present invention relates generally to dentistry and particularly to fabrication by dental technicians of dental models for bridges, splints and the like.
2. Description of the Prior Art
In fabricating a dental bridge, it is common for a dentist to prepare teeth that are to receive the bridge by grinding the teeth to a generally upwardly tapered shape. A impression of the prepared teeth is then made by the dentist and typically sent to a dental technician for fabrication of a bridge to fit the prepared teeth. The technician usually fills the impression with gypsum or another similar material to form a plaster cast of the patient's teeth and a set of parallel typically tapered metal dowel pins is then secured in the bottom of the cast with each individual pin underlying each prepared tooth and encased in an individual sleeve. The cast is then mounted atop a wet plaster base with the parallel pins extending through the base. According to one commonly used technique, when the base dries, the bottom of the base is buffed or sanded so that the pin ends are flush with the base bottom. According to another technique, some other pin and sleeve designs contain a removable cap at the bottom of each sleeve so that the removal of the cap exposes the lower part of the pin. The cast is then cut into segments called working model dies with a small saw with each segment or die bearing a cast of a prepared tooth. The model dies can thus be individually removed from the base by pressing their corresponding dowel pins upwardly from the bottom of the base. The segmented cast mounted to the base is then usually covered with wax to form a pattern wax cast copying the pattern of the teeth. When this wax pattern dries, the segmented waxed cast can be separated from the base by either pulling it manually off the set of dies or, alternately, by pressing the tapered dowel pins upwardly from the bottom of the base after which the individual working dies can be removed with the wax from the base as a unit for subsequent individual separation with the resulting wax pattern being used to cast a bridge or a crown. The second method is much more accurate since it avoids a potential distortion in the wax cast resulting from the fact that individual teeth may have converging central axis directions so that removal of a single piece wax cast is geometrically impossible without warping.
Up until the present time, the tapered dowel pins have been pressed from the base by hand to separate the waxed cast from the base using a sharp object. Since wax material remains somewhat soft, in some instances the pins are not dislodged from the holes in the base simultaneously, so that the second potential problem with wax cast removal arises in that some of the working dies are separated from the base before others which consequently results in distortion or warping of the wax pattern and creating a defect known as "open margins". A bridge cast from such a distorted pattern also bears the distortion such that it might not fit the patient's teeth properly. In this event, a new bridge often must be fabricated from scratch. Since it is difficult to determine that a pattern has been warped until the finished bridge is completed, many hours of tedious work molding the bridge from the warped impression is sometimes sacrificed.
There are many U.S. patents in the prior art relating to means for positioning and removal of the dowel pin when making a dental model. U.S. Pat. No. 2,851,728 shows a dental dowel pin having a single hole therein for receiving an elongated, rod-like repositioning gauge supported in the base stone of a dental model. The single rod-like gauge passes through more than one dowel pin, a construction which has limited practical value compared to an individual locking device for each dowel pin.
U.S. Pat. Nos. 3,413,725; 3,454,256; and 3,521,354 merely disclose dowel positioning systems, and also show the use of channel forming members located on the end of a dowel pin to form a channel in the base stone for facilitating the removal of a selected tooth die from the stone.
U.S. Pat. No. 4,457,709 shows a coiled wire rod for holding a dowel-pin in position in a dental cavity of a tooth impression during the pouring of dental die casting material into the cavity.
U.S. Pat. No. 3,896,548 shows a dental model provided with horizontal wedges which are inserted in mating sockets spanning the parting lines between adjacent tooth dies for maintaining alignment of the tooth dies within the model.
U.S. Pat. Nos. 4,056,585 and 4,139,943 show dowel pin constructions for use in a dental die.
U.S. Pat. No. 4,997,370 describes a two-piece metal dowel pin which prevents rotation of a tooth die but having an index which is invisible after the model has been trimmed.
U.S. Pat. Nos. 5,222,891 and 5,286,191 as well as Des. 303,709 and Des. 303,836 describe a dowel pin having a horizontally extending member available for aiding in die removal process. Although a useful addition, it still does not address the main problem which is a need for a simultaneous release of all dies together with a wax cast as a single unit.
U.S. Pat. No. 5,342,696 depicts a blank for a production of a dental mould part such as a crown and alike and discloses a an ejecting rod for separating the blank from the engagement groove secured by a biased ball member.
Finally, U.S. Pat. No. 4,954,081 discloses an apparatus and a method for dowel pin removal containing a movable base holding plate and a set of ejecting pins positionable under the dowel pin arrangement of the dental model. Once all ejecting pins are in place, the apparatus contains provisions allowing for lowering the base onto the set of ejecting pins thus causing model dies to separate from the base all at the same time. This apparatus is definitely useful in solving the above mentioned problem. However, correct positioning of all ejecting pins is a complicated and time consuming procedure. Complexities of design, setting up and using of this device would limit its applicability and wide spread use.
A long felt and unaddressed need still exists, therefore, for a simple hand tool die ejector aiding in simultaneous separating of the waxed dies in a dental cast from their plaster base such that the resulting wax pattern does not become warped or distorted with open margins. It is to the provision of such a die ejector tool that the present invention is primarily directed.